1. Technical Field
The invention relates to the storage and playback of still images and video segments on a display device for a digital computer, and more particularly relates to a system and a method of measuring fidelity to an original image of an image reconstructed after compression and decompression of a digital video carrying the original image.
2. Description of the Related Art
An image is reproduced on a display device such as a color monitor for a computer by the selective assignment of color and luminance to each of a matrix of picture elements or "pixels" on the display surface of the monitor. A typical matrix of pixels may have 320 columns and 240 rows for a total of 78,800 pixels. A pixel is the minimum unit which is assigned a luminance intensity, and on color monitors, a color, for the image. Video, akin to television, may be produced on a monitor by the rapid replacement or update of image frames to simulate the appearance of motion to a human observer.
As suggested by the large number of pixels on a display surface, and the need qo assign a color and luminance intensity to each pixel to make an image, the storage and transmission of digitized video data requires large capacity storage devices and large bandwidth data transmission channels. To reduce the required storage capacity of storage devices and to reduce the required bandwidth of transmission channels, various data compression techniques are routinely applied to digitized video data.
Data compression can be based on eliminating redundant information from frame to frame in a digitized video segment, on eliminating redundant information from pixel to pixel in one frame, or by exploiting superior human perception of luminance intensity detail over color detail. The International Radio Consultative Committee (CCIR) in its Recommendation 601 called for digital coding or color images based on use of an explicit luminance intensity component (Y) and two color difference components (e.g. C.sub.R and C.sub.B for the red and blue difference signals, respectively). For compression, the coding methodology applies a four to one spatial subsampling of each of the color components. Non-overlapping two by two pixel regions of a digitized frame are encoded using an individual luminance value for each pixel and two color difference values representative of the four pixel region. The methodology results in representation of four pixels with six pieces of information, or an average of one and one half pieces per pixel. In common three color coding schemes three pieces of information have been required for each pixel. Using the technique recommended by the CCIR results in compression of up to 50%. Such a technique is called "lossy" since it does not fully preserve all of the information originally captured from a scene. The CCIR 601 Recommendation is one of several known lossy methods for the compression of video data.
Different compression techniques can exhibit superior results for given images. A preferred choice is one for which most people find the reconstructed image closest to the original image. For example one technique may work better for a high textured image while another is good for a video segment including moving objects. Objective evaluation of compression methods has been hampered by lack of scoring systems which weight features that human observers tend to notice. Most untrained human observers will look more closely at the center of a picture, rather than its edges, will respond more to changes in luminance intensity than to changes in color, and will be drawn to foreground rather than background aspects of a scene. A tunable scoring system for reconstructed images which provides tools to reflect human responses is thus a desirable development.
Previously, the root mean square pixel error between values for pixel locations given by the original and reconstructed frames was used to quantify fidelity. This approach does not account for spatial errors and context, nor does it incorporate any aspect of human perception which should ideally influence how pixel differences should be penalized.